Obviously in the case of major structural collapse in the building or any other event where there is an immediate threat, the queue would be evacuated through the large gates just before the entrance to the building, or via the padlocked gate near the retail shop or through the main entrance. Potentially the entire queue can be evacuated in around 2 minutes.
If the queueline was being evacuated because of a power cut or extended shut down, half the queue would go through the building and out through fast track, the other half out the entrance of the queueline. The evacuation procedure of both the queueline and the ride itself changes depending on the reason for evacuation.
Other reasons why people are not allowed to queue in the lower part of the building are 1) because people queueing on stairs can be potentially dangerous, especially in the dark and 2) fast track is mixed with the main queue at the end of the first corridor which is far easier if the queue isn't past the fast track entrance.